From DE 44 02 653 C2, a hydraulic lifting device has become known, in which at least one hydraulic lifting cylinder is actuated by a pump in the load lifting operation mode, the pump itself being driven by an electric machine. In the operation mode of lowering the load, the pump acts as a hydraulic motor and drives the electric machine as a generator (collectible load-lowering). In the lowering branch, a hydraulic volume flow delimiter is disposed, on which the overall hydraulic pressure drops at certain operating points and which takes on the load-keeping. The lowering branch is connected between a check valve and the entry of the hydraulic pump. At least one secondary consumer is provided, which can be supplied with the medium of the lifting cylinder in the load lowering operation of the lifting cylinder. The electric machine is always operated in the same sense of rotation, and the further hydraulic consumer can be supplied directly with hydraulic energy from the lowering action, so that losses of efficiency are minimised. A rotation speed control device provides for load-independent control of the lowering speed, both in the operation of the electric machine as a generator as well as upon control solely by means of a valve arrangement. The speed of lifting as well as that of lowering results from a rotation speed control of the electric engine. In doing so, it may occur that the lowering sped becomes undesirably high when the rotation speed of the electric machine is accordingly increased, because of the high volume flow requirement of the secondary consumer.
A similar lifting device as that one described has become known from DE 100 10 870 C2. A choker valve and a pressure balance are disposed in the lowering branch, and a connection point between choker valve and pressure balance is connected to the entrance of the hydraulic pump, whilst the exit of the pressure balance is conducted to the tank. With the described lifting device, even in the double supply service, i.e. energy recovery on the lowering act and supply of secondary consumers, a relatively good efficiency is achieved.
From DE 299 11 686 U, an electro-hydraulic lifting device has become known, in which on the one hand, the volume flows to the lifting cylinder and the tank, respectively, and to secondary consumers on the other hand, are subdivided in the pressure pipe, with the aid of a proportional three-way flow control device. In this connection, it is disadvantageous that the lifting valve is disposed after the flow control device, which may also act as a priority valve, in the tank conduit. Through this, flow losses are generated in the load lowering mode.
The invention is based on the objective to provide a hydraulic lifting device for battery-operated industrial trucks, through which minimal flow losses can be achieved independently from the particular volume flow requirements.